It is well known in the photographic arts to record color images with photographic elements containing dye-providing materials which can be used to provide color images. Although the properties of dyes commonly used to provide such images (e.g. azo or azo-methine dyes) have been optimized over the years, there is a continued search in the art for dyes which provide images having improved stability to heat, humidity and chemical reagents.
Image formation based on metal chelate formation has generally been favorably regarded. The properties of the metal-ligand complexes can be manipulated by changes in both the metal and the complexing ligand. Also, metal complex dyes as a class are considered to have exceptional stability. Complexes of ferrous ions and various chromophore ligands are known to be quite stable, some having formation constants (pK) of from about 13 to about 24. Some of these complexes have been traditionally used in analytical chemistry procedures where mere color formation is important rather than a particular color hue or speed of color formation.
Iron complexes have also been used in imaging processes, for example to prepare "blue prints." In U.S. Pat. No. 1,776,155 (issued Sept. 16, 1930 to Kogel), photographic images are obtained using light-sensitive ferric salts which are reduced upon exposure to light. The resulting ferrous ions complex with certain ketones to provide a bluish color image. These elements, however, suffer from poor speed, meaning that they are not light sensitive enough for modern photographic uses. Similar light sensitive materials are described in U.S. Pat. No. 2,264,334 (issued Dec. 2, 1941 to Schmidt).
U.S. Pat. No. 3,660,092 (issued May 2, 1972 to Frank et al) relates to formation of color images in photographic elements using heavy metal salt-dye complexes. Heavy metal salts useful in the described elements include iron salts among many others. In the embodiment using iron salts, a silver halide image is first converted to a mercury salt image which is then converted to an iron salt image which releases iron to react with a ligand to form a color dye image. This imaging process, however, has several disadvantages. The use of iron complexes also requires the use of mercury in the reaction sequence. Mercury is a potential contaminent in photographic systems and should be avoided if possible. Further, the imaging process described in this reference is based on the conversion of silver halide to a metal complex, and involves a complex series of processing steps to obtain a negative image.
It would, therefore, be desirable to form highly stable color images formed with dye precursors which are essentially colorless prior to imagewise exposure. It is also desired that the process providing such images would be simple and exhibit high sensitivity (i.e. good speed) to exposing actinic radiation.